After the 1991 Persian Gulf War, wherein the tracking and mid-air destruction of SCUD missiles was a high priority, it became clear that faster and more accurate systems with broader range were required for tracking air-born threats. Since then, efforts in this area have focused on the transition from a platform-centered force to a network-centered force. The primary tenet of a network-centered force is the use of mutually-shared information to provide all members of a distributed force with timely and accurate data, thus enabling the most efficient response to threats as opposed to past operations where individual assets were deployed to respond to threats.
One program to develop network-centered technology by the United Stated Department of Defense is the Joint Composite Tracking Network (“JCTN”). The JCTN concept involves composite tracking and cooperative engagement of targets, especially theatre ballistic missiles, such as the SCUD's experienced in the Gulf War. One aim of the JTCN is to expand existing weapons systems' defended areas to enable threat engagements that were not previously possible. Composite tracking is the combined filtering of measurements from separate sensors and/or mathematical fusion of separate sensor tracks to produce, for example, estimates of position and velocity of a target that should be more accurate over a greater range than the estimate from a single sensor.
Because each sensor in a network-centered tracking system has it's own inaccuracies, a critical function of such a system is the proper assessment of the inaccuracy of the tracking solution resulting from the combined sensor data. Covariance matricies are typically used to quantify these inaccuracies.
Efficient use of battlefield communications resources requires that the covariance matricies be compressed before transmission in order to allow maximum data throughput in a minimum amount of time. Various methods may be employed to compress this data, with the goal being to maintain the accuracy of covariance matrix information as fully as possible through the compression and subsequent expansion by the receiving device.
Characteristics of the covariance matrices can help reduce the number of data bits used to represent the matrix. Since covariance matrices are symmetric, the redundant information found in off-diagonal elements can be minimized. Additionally, logarithmic encoding and severe bit truncation can also be used to reduce the data burden. Extreme truncation techniques and rounding, however, can distort the covariance matrices so that the compacted covariance matrix may bound the true covariance in one direction, but under-estimate it in another direction. It is generally considered safer to overestimate the covariance even though this discounts some of the available accuracy of a measurement. Covariance underestimation can lead to invalid tracking results. Typically, an encoding scheme is used and the resulting intermediate covariance matrix is compared to the original covariance. If the intermediate covariance matrix underestimates the original in some direction, the deficiency is calculated and the intermediate matrix is inflated and then re-encoded.
Encoding schemes such as the one proposed in the JCTN assume that range, bearing, and elevation on a track are being reported. A Cartesian covariance encoding oriented with an axis aligned in the range direction is sent with the measurement. In this configuration, the measurement information has small correlation coefficients between range and cross-range axes. These cases tend to behave fairly well with the prior art JCTN matrix encoding.
The JCTN encoding scheme encodes the diagonal elements of an original covariance matrix on a logarithmic scale, while linear encoding is used to encode the correlation coefficients of the off-diagonal elements. The encoded matrix is then expanded and compared to the original matrix. If the expanded matrix does not cover the entire original then the expanded matrix is inflated so that it does. The inflated expanded matrix is then encoded and transmitted.
Not all network-centered defense programs, pass measurement information in range, bearing and elevation coordinates. A problem with reporting in this format is that location data for the sensor source must also be transmitted. This can compromise the security of the sensor platform. An alternative system (JWIN) passes measurement and tracklet information in Earth Centered Earth Fixed (ECEF) coordinates. This allows different coalition members to report tracks without disclosing their locations. Reporting ECEF coordinates rather than own-ship-based range, bearing and elevation means that there may be significant correlations between covariance components. These correlations make encoding the covariance of a measurement more sensitive to rounding errors, and when the covariances are transmitted using the JCTN encoding scheme, there is a tendency to over-inflate the encoded covariance matrix. This results in an overstatement of uncertainty and reduces the value of the tracking data.
The need exists for a covariance encoding and compression scheme that minimizes the number of bits sent and minimizes the inflation due to the encoding while bounding the true covariance.